Oncogenes, Proto-Oncogenes, and Tumor Suppressor Genes

Oncogenes, Proto-oncogenes, and Tumor Suppressor Genes

Introduction

Cancer cells

uncontrolled cellular proliferation: lost ability to respond to signals; no longer require growth factors; normal cells exhibit a life span while cancer cells are immortalized
changes in cell surface or in cell cytoskeleton: lost anchorage dependent growth (normal cells need to attach to surface); lost tight attachment to surfaces (normal cells attach very tightly to surfaces); actin filaments are disrupted; lose contact inhibition

At cellular level, cancer is a genetic disease

Target genes for these rare mutations are a relatively small # Þ genes which control cellular proliferation

Need more than one mutant gene to produce a cancer cell

Inaccurate biochem, chemical mutagens, ionizing radiation, RNA tumor viruses Þ all lead to tumors

There is no real genetic program involved Þ similar to a natural selection process Þ formation of cancer over time is inevitable Þ randomly going to occur (random events that occur which have been selected for)

Oncogenes: involved in (+) regulation of cell growth and division

Proto-oncogenes: oncogene’s normal cellular counterparts

Tumor suppressor genes: (-) regulation of cell growth

Retroviruses

Genome and life cycle

Rous sarcoma virus: RNA tumor virus; well known causative agent of certain tumors in chickens

Retrovirus: RNA packaged in a virion containing the protein products of 3 viral genes required for viral replication

gag (group specific antigen proteins) complex with the RNA genome Þ viral core

env (envelope proteins) Þ glycoproteins

pol (reverse transcriptase) Þ RNA dependent DNA polymerase

RNA Þ DNA and integrating hosts genome randomly (uses cell’s machinery to replicate)

Transcription occurs due to strong promoter and enhancer elements found as direct repeats at the ends of the DNA copy of the viral genome

DNA genome Þ [ LTR ][ gag ][ pol ][ env ][ LTR ]

LTR not found in viral RNA genome, generated by reverse transcriptase during viral replication

After transcription, new virions assembled and released from the cell (non-destructive process)

Acutely transforming retroviruses

Short latency: cause tumors in animals while exhibiting a short lag time (weeks)

The genomes of these viruses revealed RNA sequences in addition to the gag, pol, and env genes

The extra sequence encode oncogenes responsible for the tumors produced by the virus
The oncogene of Rous Sarcoma viruses = "v-src"

Normal cells were found to contain DNA sequences related to the viral oncogenes

Later found that the retroviral oncogenes are derived from normal cellular genes

These cellular genes have been picked up by the virus due to recombination btwn the proviral and host cell genomes during the normal viral life cycle Þ called transduction Þ rare

Viral oncogenes are mutated versions of their normal cellular counterparts (proto-oncogenes) and encode proteins with abnormal and unregulated activity

Long latency retroviruses

Cause tumors after lengthy lag times following infection; genomes of these viruses lack oncogenes
Tumorigenesis occurs only when provirus integrates in/near particular host cell sequences (cellular proto-oncogenes)
When provirus integrates near/in these cellular proto-oncogenes, they convert to oncogenes

Also causes neoplastic transformation of the cell

Retroviruses and human cancer

Very few human tumors are known to result from retroviruses

Promoter insertion event

c-nyc: proto-oncogene Þ oncogene (cancerous) if viral gene (with LTR promoters) inserts upstream of c-nyc in correct orientation

Other Methods of Identification of Oncogenes

A separate class of virally encoded oncogenes are carried by certain DNA tumor viruses

often larger and more complex than retroviruses (ie sv40, polyoma, adenoviruses) Þ have been shown to interact with cellular tumor suppressor genes

Since human tumors are not viral in origin, other approaches have been taken to identify genes involved in the production of human tumors

Genomic DNA from human cancer cells (bladder carcinoma DNA) can be used in DNA-mediated gene transfer (transfection) experiments to cause neoplastic transformation of recipient mouse cells

Able to identify the oncogene responsible for the transformation event Þ identify Alu ~10⁶/genome (in human, not mouse)

Classes and functions of Proto-oncogenes

Only a small # of genes can serve as oncogenes Þ genes involved in cell growth control (in signal transduction pathway)

Growth factor receptor has 3 domains: ligand binding domain; memb spanning domain; effector domain

ser/thr/tyr can be phosphorylated

Protein tyrosine kinase domain Þ cascade events

(1) Growth Factors

one oncogene identified Þ c-sis: platelet derived growth factor

(2) Growth Factor Receptors

three regions: ligand binding domain, transmembrane domain, and protein kinase activity
protein kinase: catalyzes transfer of phosphate from ATP to another protein
ex: c-erbB; epidermal growth factor receptor

(3) Intracellular Transducers

cytoplasmic protein kinases (ex: c-src: protein tyrosine kinase)

ser/thr phosphorylation usually prevalent but tyr kinases are greatly elevated in cells transformed by oncogenes

G-proteins (GTPases): GTP Þ GDP

When GTP is bound, activates protein kinase

Ras proteins

Autoregulated thru their own GTPase activity (GTP- active ras Þ GDP- inactive ras)

GAPs (GTP activator proteins): stimulate ras GTPase activity; (-) regulator of ras

GNRPs (guanine nucleotide releasing proteins): exchanges GTP for GDP bound to ras; (+) regulator of ras

(4) Nuclear Transcription Factors

nuclear proteins acting as transcription factors for genes: c-fos; c-jun; c-myc (associated with human tumors)
stimulates transcription from genes with appropriate promoters/enhancers

Types of Genetic Lesions Converting Proto-oncogenes to Oncogenes

Proto-oncogene Þ oncogene; via dominant mutation (gain of function)

(1) single a.a. substitutions

ras gene: ß in GTPase activity; Ý in ras signalling activity; keeps ras in "on" state (proto-onc Þ onc); gain of function activity; common mutation in causing human cancers

(2) deletion mutation

erbB (epidermal growth factor receptors – EGF)

if ligand binding domain at N-terminus is deleted: consititutively active, always "on"

Similar to animal systems when a provirus is inserted btwn the LBD and TM Þ unregulated protein kinase activity

(3) Translocation

c-abl = encodes a ser/thr kinase Þ activated due to translocation

chronic myo leukemia Þ Philadelphia translocation (9-22)

Burkitt’s lymphoma (c-myc)

Translocation puts c-myc by gene making immunoglobulins (makes Ig at high rates)
Turns c-myc on to a very high level

(4) gene amplification

over expression of oncogene products (like myc) and ras genes

These are not programmed mutations, but random mutations which inevitably occur and become selected (statistical inevitability)

Tumor Suppressor Genes

Rb gene: regulates transcription in cellular proliferation (tumor suppressor gene)

loss of function Þ proto-oncogene Þ oncogene (genetic recessive function)

P53: transcription factor Þ for genes whose proteins negatively regulate the cyclins and CDKs

Wilm’s tumor

NF1: looks like its regulated to gaps - Ý GTPase activity which ß ras signaling activity

Multiple Genetic Lesions Required for Carcinogenesis

Oncogene cooperation; a single oncogenic activation of a ras gene is not enough to produce a tumor in a mouse Þ but makes them way more susceptible to becoming cancerous

Need multiple mutations for carcinogenesis (3-7); phorbol esters